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FreeBSD-SA-24:10.bhyve                                      Security Advisory
                                                          The FreeBSD Project

Topic:          bhyve(8) privileged guest escape via TPM device passthrough

Category:       core
Module:         bhyve
Announced:      2024-09-04
Credits:        Synacktiv
Sponsored by:   The FreeBSD Foundation, The Alpha-Omega Project
Affects:        FreeBSD 14.x
Corrected:      2024-09-04 15:42:29 UTC (stable/14, 14.1-STABLE)
                2024-09-04 21:07:28 UTC (releng/14.1, 14.1-RELEASE-p4)
                2024-09-04 20:54:13 UTC (releng/14.0, 14.0-RELEASE-p10)
CVE Name:       CVE-2024-41928

For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the
following sections, please visit <URL:https://security.FreeBSD.org/>.

I.   Background

bhyve(8) is a hypervisor that runs guest operating systems inside a virtual
machine.

II.  Problem Description

bhyve can be configured to provide access to the host's TPM device, where it
passes the communication through an emulated device provided to the guest. This
may be performed on the command-line by starting bhyve with the
`-l tpm,passthru,/dev/tpmX` parameters.

The MMIO handler for the emulated device did not validate the offset and size
of the memory access correctly, allowing guests to read and write memory
contents outside of the memory area effectively allocated.

III. Impact

Malicious software running in a guest VM can exploit the buffer overflow to
achieve code execution on the host in the bhyve userspace process, which
typically runs as root. Note that bhyve runs in a Capsicum sandbox, so
malicious code is constrained by the capabilities available to the bhyve
process.

IV.  Workaround

No workaround is available, but guests that do not use TPM passthrough are
not impacted.

V.   Solution

Upgrade your vulnerable system to a supported FreeBSD stable or
release / security branch (releng) dated after the correction date.

Guest operating systems exposing the TPM device need to be restarted for the
correction to be applied. (i.e., their corresponding bhyve process needs to be
terminated and started again)

Perform one of the following:

1) To update your vulnerable system via a binary patch:

Systems running a RELEASE version of FreeBSD on the amd64 or arm64 platforms,
or the i386 platform on FreeBSD 13, can be updated via the freebsd-update(8)
utility:

# freebsd-update fetch
# freebsd-update install

2) To update your vulnerable system via a source code patch:

The following patches have been verified to apply to the applicable
FreeBSD release branches.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

# fetch https://security.FreeBSD.org/patches/SA-24:10/bhyve.patch
# fetch https://security.FreeBSD.org/patches/SA-24:10/bhyve.patch.asc
# gpg --verify bhyve.patch.asc

b) Apply the patch.  Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch

c) Recompile the operating system using buildworld and installworld as
described in <URL:https://www.FreeBSD.org/handbook/makeworld.html>.

Restart the corresponding bhyve processes, or reboot the system.

VI.  Correction details

This issue is corrected as of the corresponding Git commit hash in the
following stable and release branches:

Branch/path                             Hash                     Revision
- -------------------------------------------------------------------------
stable/14/                              6ce4821f0859    stable/14-n268656
releng/14.1/                            eab723be7542  releng/14.1-n267697
releng/14.0/                            429f200688ca  releng/14.0-n265434
- -------------------------------------------------------------------------

Run the following command to see which files were modified by a
particular commit:

# git show --stat <commit hash>

Or visit the following URL, replacing NNNNNN with the hash:

<URL:https://cgit.freebsd.org/src/commit/?id=NNNNNN>

To determine the commit count in a working tree (for comparison against
nNNNNNN in the table above), run:

# git rev-list --count --first-parent HEAD

VII. References

The corresponding part of the security audit report as provided by Synacktiv
will be published in due course.

<URL:https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2024-41928>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-24:10.bhyve.asc>
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